1. Field of the Invention
The present invention relates to a semiconductor device used for an ignition device for an internal combustion engine of a car. Particularly, it relates to a semiconductor device which includes a power semiconductor element serving as a switch and which is provided with a function of pulling down a gate of the power semiconductor element.
2. Description of the Background Art
A semiconductor device including a power semiconductor element which controls switching of a primary-side current of an ignition coil is used for an ignition device for an internal combustion engine of a car. FIG. 7 shows an example of a configuration of an ignition semiconductor device for an internal combustion engine according to the background art, in which an IGBT (Insulated Gate Bipolar Transistor) is used as a power semiconductor element. An ignition semiconductor device 700 which includes an engine control unit (ECU) 701, an ignition semiconductor integrated circuit (IC) 702, an ignition coil 703, a voltage source 704, and a spark plug 705 is shown in FIG. 7.
The ignition IC 702 includes an IGBT 723 which controls ON/OFF of a primary current of the ignition coil 703, a gate resistor 721, a Zener diode 722 which clamps a collector voltage of the IGBT 723, a C terminal (collector electrode) which is connected to one end of the ignition coil 703, an E terminal (emitter electrode) which is connected to a ground potential, and a G terminal which serves as an input terminal connected to the ECU 701. Accordingly, the ignition IC 702 has three terminals, i.e. the C terminal, the E terminal and the G terminal, as its terminals.
Operation of the ignition semiconductor device 700 shown in FIG. 7 will be described here. The ECU 701 outputs a signal for controlling ON/OFF of the IGBT 723 of the ignition IC 702 to the G terminal. When, for example, 5 V is inputted to the G terminal from the ECU 701, the IGBT 723 turns ON. When 0 V is inputted to the G terminal, the IGBT 723 turns OFF.
Next, an operation waveform of the ignition semiconductor device 700 shown in FIG. 7 will be described with reference to FIGS. 3A to 3C. First, an ON signal VG of an H (high) level is inputted to the G terminal from the ECU 701 as shown in FIG. 3C, the IGBT 723 turns ON and a collector current Ic begins to flow into the C terminal from the voltage source 704 (for example, 14 V) through a primary coil 731 of the ignition coil 703 (time instant t1). As to the collector current Ic, dI/dt is determined based on the inductance of the primary coil 731 and the voltage applied thereto. Next, when an OFF signal VG of an L (low) level is inputted to the G terminal from the ECU 701, the IGBT 723 turns OFF and the collector current Ic decreases suddenly (time instant t2). Due to the sudden change of the collector current Ic, a voltage between opposite ends of the primary coil 731 increases suddenly. At the same time, a voltage between opposite ends of a secondary coil 732 of the ignition coil 703 also increases up to several tens kV (for example, 30 kV) and this voltage is applied to the spark plug 705. The spark plug 705 discharges electricity when the applied voltage is at least about 10 kV.
Further, operation for about 10 microseconds immediately after the time instant t2 will be described in detail. When a collector voltage Vc of the IGBT 723 exceeds a withstand voltage Vzd of the Zener diode 722, a current flows into the gate resistor 721 through the Zener diode 722. Thus, a gate voltage VGout and the collector voltage Vc of the IGBT 723 can keep the relation VGout≈Vth, and the relation Vc≈Vzd unless energy release of the primary coil 731 is completed. When the energy release of the primary coil 731 is completed, the gate voltage VGout and the collector voltage Vc of the IGBT 723 decrease until the gate voltage VGout is equal to 0 V and the collector voltage Vc is equal to VB. Here, the reference sign Vth designates a threshold voltage of the IGBT 723 and the reference sign VB designates a power supply voltage. The aforementioned relation VGout≈Vth is a condition on which the IGBT 723 cannot be turned ON fully to make the collector voltage Vc substantially equal to 0 V but the IGBT 723 can be turned ON slightly enough to prevent the collector voltage Vc from reaching the withstand voltage Vzd or higher.